The mTOR kinase is the central component of a pathway controls growth in eukaryotes and is deregulated in common human diseases like cancer and diabetes. Within cells mTOR exists within two distinct protein complexes, mTOR Complex 1 (mTORC1) and 2 (mTORC2). mTORC1 contains mTOR, mLST8/G2L, raptor, and PRAS40, is partially sensitive to rapamycin, and controls cell size through translational regulators like S6K1 and 4E-BP1. mTORC2 also contains mTOR and mLST8, but, instead of raptor and PRAS40, it contains rictor, mSin1, and protor. We know less about mTORC2 than mTORC1 but it is now accepted that mTORC2 is an activating kinase for Akt/PKB and SGK and therefore part of the PI3K pathway that controls cell survival, proliferation, and metabolism. Recently, we discovered that DEPTOR, a protein of previously unknown function, interacts directly with mTOR and inhibits mTORC1 and mTORC2 signaling in cells through unclear mechanisms. DEPTOR protein levels are highly regulated by the same growth stimuli and stresses that regulate mTORC1 and mTORC2. Overexpression of DEPTOR inhibits mTORC1 signaling, which, in turn, activates the PI3K pathway by suppressing a known inhibitory feedback from mTORC1 to PI3K. In cancers like Multiple Myeloma, DEPTOR is highly overexpressed and the resulting activation of PI3K is a new mechanism for promoting cell survival. We propose to: (1) determine how DEPTOR inhibits mTORC1 and mTORC2 signaling; (2) identify and characterize the molecular mechanisms that regulate the expression level of DEPTOR; and (3) determine the in vivo role of DEPTOR in the mTORC1 and mTORC1 pathways and in controlling liver growth and function. We will accomplish our goals with a multi-disciplinary approach that uses the tools of biochemistry, molecular biology, proteomics, high-throughput RNAi screening, and engineered mouse models. Our results are likely to have important consequences for our understanding of the clinically important mTOR pathway and the signaling mechanisms we uncover may serve in the future as targets for drug development.